You need to balance school, work, and extra-curricular activities. If you are really struggling with your classes, you may want to give up the afterschool job or an extra-curricular activity until your grades come up. You need to prioritize your time. Remember: your education is the most important thing.[2]
For college classes, you should base the hours you study per class on how difficult the class is and how many credit hours the class is worth. For example, if you have a 3 hour physics class that is really hard, you want to study 9 hours a week (3 hrs x 3 for hard difficulty). If you have a literature course that is worth 3 hours and is kinda hard, you may want to study 6 hours a week (3 hrs x 2 for medium difficulty).[3]